Probably not an awesome idea to have a way for light to get in like that, but in a real shooting situation, I seriously doubt this would be an issue. I suppose the youtuber just needs some more video hits or something.

Not all long exposures are done in low light. I frequently shoot moving water, such as streams and waterfalls, using very dark ND filters and exposures in the 4-10 second range. Experimenting with my camera, which exhibits the problem, and just using a table lamp as a light source to the side of the camera, I get a purple spot of light on one edge. That is with an exposure of 8 seconds at ISO 200. These are very realistic exposure values.

To be sure, I would probably be controlling the camera with my phone, rather than a cable. But not everyone would use a phone. For those using a cable, a dummy plug in the unused port would be a good idea.

I can work around this problem easily enough. But, if I had had a choice of buying a camera that had the problem and one that didn't, I certainly would have picked the one that didn't. I can't imagine anyone else doing differently.